Although we have yet to detect an alien ping, improvements in technology have encouraged us to think that, if transmitting extraterrestrials are out there, we might soon find them. That would be revolutionary. But some, Stephen Hawking included, sense catastrophe.

Hawking is worried about aliens. The famous physicist recently suggested that we should be wary of contact with extraterrestrials, citing what happened to Native Americans when Europeans landed on their shores. Since any species that could visit us would be far beyond our own technological level, meeting them could be bad news.

Hawking was extrapolating the possible consequences of my day job: a small but durable exercise known as SETI, the Search for Extraterrestrial Intelligence.

Consider what happens if we succeed. Should we respond? Any broadcast could blow Earth's cover, inviting the possibility of attack by a society advanced enough to pick up our signals.

On the face of it, that sounds like a scenario straight out of cheap science fiction. But even if the odds of calamity are small, why gamble?

For three years, this issue has been exercising a group of SETI scientists in the International Academy of Astronautics. The crux of the dispute was an initiative by a few members to proscribe any broadcasts to aliens, whether or not we receive a signal first.

In truth, banning broadcasts would be impractical - and manifestly too late. We have been inadvertently betraying our presence for 60 years with our television, radio and radar transmissions. The earliest episodes of I Love Lucy have washed over 6000 or so star systems, and are reaching new audiences at the rate of one solar system a day. If there are sentient beings out there, the signals will reach them.

Detecting this leakage radiation won't be that difficult. Its intensity decreases with the square of the distance, but even if the nearest aliens were 1000 light years away, they would still be able to detect it as long as their antenna technology was a century or two ahead of ours.

This makes it specious to suggest that we should ban deliberate messages on the grounds that they would be more powerful than our leaked signals. Only a society close to our level of development would be able to pick up an intentional broadcast while failing to notice TV and radar. And a society at our level is no threat.

The flip side is that for any alien society that could be dangerous, a deliberate message makes no difference. Such a society could use its own star as a gravitational lens, and even see the glow from our street lamps. Hawking's warning is irrelevant.

Such considerations motivated the SETI group at the International Academy of Astronautics to reject a proscription of transmissions to the sky. It was the right decision. The extraterrestrials may be out there, and we might learn much by discovering them, but it is paranoia of a rare sort that would shutter the Earth out of fear that they might discover us.

Seth Shostak is senior astronomer at the SETI Institute, and chair of the International Academy of Astronautics' SETI Permanent Study GroupNew ScientistNot just a website! Subscribe to New Scientist and get: New Scientist magazine delivered to your door Unlimited online access to articles from over 500 back issues Subscribe Now and Save